German authorities are working on the assumption that the truck that steered into the crowd in a Christmas Market in Berlin on Monday evening was a terrorist attack. At about 8 p.m. Monday, a black semitrailer with Polish license plates drove onto a Christmas Market in the center of Berlin,killing 12 people and injuring 48 additional people. The police detained the suspected driver, a Pakistani, about a mile away.

He was reportedly known to police for minor crimes, but not terror links, reports the BBC.

German media say police have searched a refugee shelter at a defunct Berlin airport where the suspect was believed to be staying.

In a short statement on Tuesday, German Chancellor Angela Merkel said those behind the attack would be punished "as harshly as the law allows". She added that it would be "particularly sickening" if he were proven to be a refugee.

In Philadelphia, visitors of the Christmas Market expressed their support to the victims in Berlin, reports CBS.

U.S. President-elect Donald Trump is blaming Islamist terrorists for the truck attack. Although German authorities are still investigating the attack, the White House earlier also said the incident "appears to have been a terrorist attack."

Trump's statement offers nothing to back up his claim that Islamist terrorists were behind the attack. He says they and the Islamic State group continually slaughter Christians in their communities and places of worship as part of their global jihad, reports The Associated Press.

The president-elect adds that terrorists must be "eradicated from the face of the earth" and pledges to carry out that mission with all "freedom-loving partners."The Islamic State group and al-Qaida have both called on followers to use trucks to attack crowds.

Several German media reports said early Tuesday that the driver had arrived in February as an asylum seeker from Pakistan or Afghanistan. The reports couldn’t be independently verified. If that is confirmed, the driver’s origin would stoke tensions over a wave of migrants from Muslim countries that has fueled nationalist sentiment and roiled politics in Germany and across Europe, reports The Wall Street Journal.

The crash came less than a month after the U.S. State Department called for caution in markets and other public places across Europe, saying extremist groups including Islamic State and al-Qaida were focusing "on the upcoming holiday season and associated events."

The Islamic State group and al-Qaida have both called on followers to use trucks in particular to attack crowds. On July 14, a truck plowed into Bastille Day revelers in the southern French city of Nice, killing 86 people. Islamic State claimed responsibility for that attack, which was carried out by a Tunisian living in France.